The invention relates to a gas discharge lamp having electrodes of a ceramic material.
A gas discharge lamp comprises a radiation-transmitting discharge vessel which encloses a discharge space containing a gaseous, ionizable filling. Suitably spaced electrodes are arranged in this discharge space.
U.S. Pat. No. 5,654,606 discloses such a gas discharge lamp. Instead of the customary metal electrodes, a sintered mixture of metal and ceramic material is used as the coupling-in structure. By generating a high capacitive voltage between the coupling-in structures, the charge carriers are generated directly in the gas volume in such gas discharge lamps. The ceramic materials used required the addition of small quantities of metal to obtain sufficiently stable electrodes at temperature variations which may occur when such a gas discharge lamp is switched on.
According to the invention the electrodes are made of a ferroelectric ceramic.
A ceramic material for such electrodes must have a (substantially) rectangular hysteresis loop, a high dielectric constant ∈r and a high remnant polarization Pr.
Most dielectric materials exhibit a low value of the dielectric constants ∈r and a small field-dependence ∈r (E). There are a few ferroelectric materials that are an exception to this rule; these materials exhibit ∈r values which demonstrate a strong, discontinuous variation at a critical field intensity Ec.
Discs of ferroelectric materials, which exhibit a so-called non-linear behavior, can be used as electrodes in gas discharge lamps. These discs act as ceramic plate capacitors, and upon applying an alternating voltage, the inner surfaces are charged. The substantial, non-linear rise of the capacitor charge brings about the ignition and the subsequent continuous operation of the lamp.
Preferably, the ferroelectric ceramic comprises Ba(Ti1xe2x88x92xZrx)O3 doped with donor/acceptor combinations.
Ba(Ti1xe2x88x92xZrx)O3 doped with donor/acceptor combinations is a ferroelectric material having the required non-linear properties. In Ba(Ti1xe2x88x92xZrx)O3 mixed crystal ceramics, small additions of donor/acceptor combinations bring about high values of the remnant polarization Pr and the dielectric constant ∈r. In addition, these donor/acceptor-doped Ba(Ti1xe2x88x92xZrx)O3 ceramics exhibit rectangular hysteresis loops.
It is preferred that the donor/acceptor combinations comprise Mn3+ and W6+ or Yb3+ and Nb5+ or Yb3+ and Mo6+ or Mg2+ and W6+ or Mn3+ and Nb5+ or Yb3+ and W6+ or Mg2+ and Nb5+ or Mn3+ and Dy3+, Ho3+, Er3+, Gd3+, Nd3+, Y3+.
These donor/acceptor combinations bring about a particularly strong rise of the values of the dielectric constants ∈r and the remnant polarization Pr.
It is also preferred that the zirconium content in the ferroelectric ceramic is x=0.09.
The addition of BaZrO3 to BaTiO3 causes the coercive field strengths in mixed crystals of the composition Ba(Ti1xe2x88x92xZrx)O3 to be reduced to Ec less than 100 V/mm. At an operating voltage of 220 V, this advantageously enables the use of coupling-in structures in a thickness such that a sufficient dielectric strength is obtained. At a zirconium content of x=0.09, the coercive field strength Ec≈70 V/mm, and the Curie temperature Tc is 90xc2x0 C., which temperature lies in a range above the operating temperature of gas discharge lamps.
It is further preferred that the ratio Ba/(Ti,Zr,dopants) lies in the range between 0.997 and 0.998.
In Perovskites, the atomic ratio between the cations has a large influence on the properties of the ceramic material. In the mixed crystal series Ba(Ti1xe2x88x92xZrx)O3, the largest increase of the dielectric constant ∈r in dependence upon Ec or Tc is obtained when the atomic ratio Ba/(Ti,Zr,dopants) is slightly smaller than 1.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment described hereinafter.